Galaxies and Pairs in a Deep WFPC2 Field
Session 106 -- Galaxies: Photometry and Spectrophotometry
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

## [106.03] Galaxies and Pairs in a Deep WFPC2 Field

W. C. Keel (U. Alabama), R. A. Windhorst and B. E. Franklin (Arizona State U.)

We assess the population of faint field galaxies in a deep WFPC2 observation centered on the radio galaxy 53W002 ($z=2.39$; see parallel poster by Windhorst \& Keel), with emphasis on the pairing and grouping properties. Images in both F606W and F814W filters were obtained, with total exposure almost 6 hours in each case. Rich detail is seen in galaxies at a wide range of apparent magnitudes and/or redshifts. The field includes no obvious rich clusters, but a variety of large-scale structures do appear along this line of sight at redshifts $z=$0.275, 0.55, 1.10, and 2.40 (see parallel poster by Pascarelle et al.).

Of particular interest is a population of very tight galaxy pairs at separations 0.3--0.5 arcsecond visible (at component magnitudes $I>24$) in both the WFC and PC chips of WFPC2. These are virtually lacking in WF/PC-1 studies of adjacent fields (Burkey, Keel, Windhorst \& Franklin 1994 ApJL 429, L13). Two such pairs appear in the PC field with 53W002 itself, and at least one member of such a pair shows Lyman $\alpha$ emission at $z=2.39$ (see parallel poster by Pascarelle et al.), making it a confirmed member of the high-redshift group or cluster around 53W002. The distribution of the tight pairs within the field suggests that a good fraction of them may be members of this cluster, strengthening suggestions that the merging rate of galaxy pairs has a strong dependence on environment. %%% I

For any reasonable redshift, the WFPC2 pairs cannot have projected separations larger than about 3 kpc. This implies a very short lifetime against merging if the members are like comparably luminous present-day galaxies, and hence that we must be seeing very short-lived phases, or more compact galaxies. %%% Check! In these extreme pairs, we may be seeing the end of a phase of galaxy building from small, dense progenitors. %%% In fore-last sentence, check my change!

This work was supported by NASA/HST grant GO-5308-01-94A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.